Design simulation is a useful technique which allows engineers to determine the functionality of circuit designs. The simulation may be performed prior to the fabrication of a physical circuit (i.e., in the pre-silicon stage) and may allow for easy tracking of the circuit functionality as the value of each wire in the design at every cycle.
In some cases, fabricated designs may encounter a hard error. A hard error is a defect that occurs at some point in time to a particular product fabricated from said design, a defect that modifies the functionality of the design. A hard error may occur as a result of a fabrication problem, or it may happen a long time after fabrication, when the product is already in use. In some cases, hard errors may be permanent errors that are not rectified even when the design is rebooted.